When you crossfire or sli, you aren't really doubling the bandwidth of your total gpu power, meaning if you crossfire in x16, x16, you don't have 32 lanes. The difference is x16, x16 is more efficient because you still have 16 lanes of pcie between the 2 cards, but those lanes are split between the cards unevenly, depending on how much use either card is getting. While other set ups would be x8, x8 permanently. This is able to work because most graphics card cannot come close to saturating the pcie slots bandwidth, so there's a lot of unused space most of the time. 550ti's are not powerful cards so they would not saturate an x4 pcie 2.0 slot, so yes, with those cards it would be worth it to sli.
550ti sli is a few fps slower than a 570, so I would say it'd be worth it to look at a 760 or 770 if those are in your price range.